xi] THE LACE-PLANT OF MADAGASCAR 143 



they merely make use of the debris of those cells which are 

 sacrificed to form the perforations. 



That the fenestration in Ouvirandra may be of some value in 

 connexion with aeration was suggested by Goebel's statement 

 that the tissue of the leaf is poor in intercellular spaces^. M'^^ 

 Sergueeff, however, shows that Goebel is in error on this point, 

 since numerous lacunae occur in the mesophyll, and she con- 

 cludes that the main function of fenestration is not aeration, 

 but the reduction of resistance to current. In this connexion it 

 may be recalled that all the imperforate, submerged leaves 

 found among the Aponogetons are thin and ribbon-shaped, 

 thus yielding easily to the movement of the water (e.g. A. angus- 

 tifolius^ Ait.)^. It may also be significant that A. (Ouvirandra) 

 fenestraliSj in its Madagascan home, though it sometimes grows 

 in stagnant water, is also capable of living in torrents. Hans- 

 girg^ had previously suggested that some forms of submerged 

 leaf might be compared with those of such ' anemophytes ' 

 among terrestrial plants as Palms, Bananas, etc., in which the 

 slitting, sub-division and perforation of the leaves are interpreted 

 by some authors as modifications designed to avoid tearing by 

 the wind. But the view that would regard all types of submerged 

 leaf as definite adaptations to water life, probably needs con- 

 siderable revision. We do not propose to criticise it at this point, 

 since it is included in the broader question of the relation of leaf 

 form to environment, which is better considered in connexion 

 with heterophylly*. 



(2) THE FACTS OF HETEROPHYLLY UNDER NATURAL 



CONDITIONS^ 



The occurrence of two or more different types of leaf upon 

 one individual, which is so frequently characteristic of water 

 plants, has long attracted the interest of botanists. 



1 Goebel, K. (i 891-1893). 2 Krause,K. and Engler, A.(i9o6) 



2 Hansgirg, A. (1903). ^ See Section (3) of this Chapter. 



^ Arber, A. (1919^) has been largely incorporated in Sections (2) and 

 (3) of this Chapter. 



